A lithium-ion secondary battery, which is a type of nonaqueous electrolyte secondary batteries, has an extremely-high electromotive force (about 3 V or more) compared to an electromotive force (about 1.5 V) of an aqueous electrolyte secondary battery (for example, nickel hydrogen battery, a nickel-cadmium battery, and a lead battery). Therefore, the lithium-ion secondary battery is advantageous in reducing size and weight of the battery and in increasing capacity and output power, and has been commercially available to compact electronic devices such as a portable personal computer and a portable phone. In recent years, an application of the lithium-ion secondary battery is expanded even to large-scale electric devices (for example, a power source for vehicle such as HEV (Hybrid Electric Vehicle) and EV (Electric Vehicle), and a power source for storing power).
There is own that degradation such as a decrease of the battery capacity and an increase of the resistance occurs in a general secondary battery according to the number of use and the number of use days. The number of use herein is defined by the number of cycles in which one cycle corresponds to one time of charging/discharging. Therefore, in a case where the battery is mounted in various types of power storage systems, there is a need to determine an initial battery performance in consideration of the degradation. It is known that a degree of degradation is proportional to a square root of the number of cycles or the number of use days with time. Therefore, when a system is designed, the battery capacity (hereinafter, referred to as “initial battery capacity”) of the secondary battery to be mounted at the initial time and the battery performance are determined in consideration of the number of cycles when system life reaches, the number of use or an accumulated charging/discharging capacity by calculating backward from the battery capacity and the battery resistance (collectively referred to as “battery performance when the life reaches”) which are necessary when the system life reaches. With this configuration, there is no need to exchange a battery during a period when the system operates, and the system can be improved in reliability.
On the other hand, in an actual use situation, degradation significantly deviates from a transition estimated at the time of designing the system by a root rule, and the degradation of the secondary battery progresses faster than designed (hereinafter, denoted as “decrease of life estimation accuracy”). These problems cause an increase in size and initial cost due to the batteries mounted more than necessary in addition to a decrease of the system reliability caused by an urgent maintenance such as a replacement work of the secondary battery without warning, an increase in a life cycle cost, and a degradation progressing faster than designed.
Regarding such problems, in PTL 1, a correlation resistance value of a positive electrode and a correlation resistance value of a negative electrode are estimated as an inner state related to a remained value such as a degradation speed and a life span of the secondary battery using transition behavior data from a use history based on the states (voltage, current, temperature, use time, etc.) of the secondary battery. A degradation balance is calculated from a ratio of the correlation resistance value of the positive electrode and the correlation resistance value of the negative electrode. In PTL 1, emphasis is placed on improvement of an output performance.